It is very hard to clearly decide which choices in the game are so significant that matters in the final endings, but rather only being a variation of one ending - at least for the purposes of developers. After many thoughts, I concluded that the most important decision is clearly presented to the player in the form of videos/animations. The following graph presents all possible paths of the completion the game The Witcher 2: Assassins of Kings.
The first important decision in the game is Aryan's fate during the mission At the Fore. This decision shapes the rest of the Prologue, while escaping from prison in mission The dungeons of the La Valettes. If you kill Aryan, his mother Louisa will help you with escaping. If Aryan survives, we will meet him, which results in burning the castle. This choice could be crucial in the future.
Next choice - event more important - is to choose one side of the conflict. Whether you take the Iorweth's or Roche's path, radically changes the course of the second and third act.
Associated with chosen path is Stennis' and Henselt's fate. In the Iorweth's path you can decide whether you allow lynching or try to stop it during mission Royal Blood. This choice can be important in the future division of the kingdom. It is worth notice that in this path Henselt capitulates. Choosing the Roche's path, you have a direct influence of Henselt's fate during the last mission in second act - Vergen Besieged. It is worth notice that in this path you find dead Stennis.
The last important choice is to choose between rescuing Triss or Filippa (Iorweth's path) / Anais (Roche's path).